The invention concerns a digital image recording method and device, in which a color toner image is transferred to an image carrier substrate and then melted and fixed by exposure to electromagnetic radiation on the image carrier substrate
A known digital recording method is electrostatic printing, in which a latent electrostatic image is developed by charged toner particles. These are transferred to an image receiving substrate, hereafter substrate for short. The developed image transferred to the substrate is then fixed, the toner particles being melted by supplying heat.
Contacting methods are often used to melt the toner particles, in which the toner particles are brought into contact with corresponding devices, for example, hot rolls or rollers. A shortcoming here is that the design, maintenance and operating costs of these contact heating devices are demanding and therefore cost intensive. The use of silicone oil as parting agent is also required, which is supposed to prevent adhesion of the melted toner to the heating device. The error rate caused by the contact heating devices is also relatively high.
For fixation of the toner transferred, for example, to paper, contactless heating devices and methods are also known, in which the toner particles are melted by means of heat/microwave radiation or with hot air, so that they adhere to the paper.
The toner image in color printing can have four toner layers of different color, in which the toner layers are ordinarily black, yellow, magenta and cyan. The maximum density of each toner layer on the image carrier substrate is 100%, so that a maximum total density of the toner layers/toner image of 400% is obtained. The density of the toner image ordinarily lies in a range from 10% to 400%. A toner layer with only 10% density is primarily formed by individual toner particles on the image carrier substrate. Since the density of the toner image can vary in a range from 10% to 400%, it has proved to be extremely difficult to melt the toner image by means of a UV radiation device that exposes the toner image to ultraviolet radiation. The reason for this is that the energy for melting of a toner image with a toner density of 10% in a toner image with a toner density of 400% leads to blistering within the toner layers, because of air inclusions and moisture emerging from the image carrier substrate and the toners. Moreover, the energy to melt the toner image with a toner density of 400% is not sufficient to melt the toner image with a toner density of 10%.
UV radiation, whose wavelength range is so wide that all color toners of the toner image absorb sufficient energy for melting, can be used to melt a toner image having a density of 400%. A shortcoming here is that the energy distribution of the absorbed energy in the overlapping toner layers is very different, for example, the first, uppermost toner layer reflects about 5% and absorbs about 80% of the radiation energy, whereas the second toner layer, lying beneath the first toner layer, absorbs only about 15% of the radiation energy. The high energy input into the uppermost toner layer of the toner image is the reason, in known image recording methods, that the so called blistering (bubble formation in the toner image) occurs in high density toner layers, which affects the luster of the toner image in undesired fashion.
It is therefore the task of the invention to provide a method and device of the type, in which sufficient melting of the toner image can be guaranteed even at high densities of the toner layers. At the same time, blistering within the toner layers should be preferably fully prevented, but at least reduced to a harmless degree.
To solve the task, a digital recording method is proposed wherein a color toner image, having at least two, preferably four, toner layers of different color, be transferred to an image carrier substrate. The image carrier substrate can be formed, for example, from a sheet or continuous web, consisting of paper or cardboard. After all toner layers have been transferred in known fashion to the image carrier substrate, the toner image is exposed in a subsequent process step to electromagnetic radiation, so that the toner layers are melted on the image carrier substrate and fixed because of this. The method is characterized by the toner layers with different colors are melted in succession. For this purpose, the toner image is repeatedly exposed to electromagnetic radiation with a different wavelength range, the corresponding wavelength range being chosen so that, in each case, one color absorbs most of this electromagnetic radiation. An advantage of the method according to the invention is that, because of layered melting of the toner image, heating of the image carrier substrate is only relatively limited, so that the water removal rate from the image carrier substrate is strongly reduced. Blistering in the toner layers can be practically ruled out because of this.
In a preferred variant, it is proposed that the toner image for each of the process colors, cyan, magenta and yellow be exposed at least once to electromagnetic radiation with different wavelength ranges. The corresponding wavelength range is adjusted to the color of the melting toner layer, so that this absorbs most of the radiation in comparison with each of the other colors of the toner image. The intensity and duration of the radiation is chosen so that the toner layer is also melted in the desired fashion, while the other toner layers are just preheated. The other color toners can optionally also absorb part of the electromagnetic radiation that is actually not intended for them at all, in which the corresponding absorption rate is lower, preferably much lower, than the rate in the toner layer that is supposed to be melted.
An advantageous variant of the process is characterized by the toner image has four toner layers of different color, and that the toner image is exposed a total of three times to electromagnetic radiation with different wavelength ranges each time. It is proposed according to the invention that separate exposure to different wavelength ranges be carried out only to melt the magenta, cyan and yellow toner layers, whereas the black toner layer is not exposed separately. The reason for this is that the black toners have such good absorption behavior in all wavelength ranges for the other different colored toner layers that exposure of the magenta, cyan and yellow toner layers is sufficient in order to heat the black toner to or above its melting or glass transition point.
According to a modification of the invention, it is proposed that the radiation energy in each of the melting processes be high enough that the toner layer is also melted at low density. The electromagnetic radiation therefore has a high enough radiation energy from the outset that both toner layers with a high density, for example, 100%, and those with low density, for example, 10%, can be uniformly melted. Determination of the corresponding toner density to adjust the radiation energy is not necessary here, which simplifies control of the image recording process.
In a preferred variant, the radiation energy for a toner layer of the toner image with a density of 10% to 100% is equally large. In this practical example, the lowest possible density of a toner layer is therefore used as a basis to determine the radiation energy required for melting of this toner layer. This means that, if the radiation energy is sufficient to melt a toner layer with a density of 10%, this radiation energy is also sufficient for toner layers having a density of up to 100%.
Alternatively, to solve the task, a digital image recording device is also proposed, which is an electrographic or electrophotographic printer or copier, having a fixation device for fixation of a toner image on an image carrier substrate, in which the image carrier substrate is ordinarily conveyed past the fixation device by a conveyor device. The device is characterized by the fixation device for each toner layer, having one of the process colors cyan, magenta or yellow, has a radiation unit, by means of which electromagnetic radiation in a specific wavelength range can be applied to the toner image. A separate radiation unit is therefore prescribed for each of these process colors, in which the wavelength of its electromagnetic radiation applied to the toner image is adjusted to the corresponding process color, so that this absorbs most of the radiation relative to the other toner colors. The advantages arising from this are that overheating of the toner image and image carrier substrate can be ruled out.
In conjunction with the present invention, xe2x80x9cradiation unitxe2x80x9d is understood to mean a heating unit to heat the toner image for fixation on the image carrier substrate, which transfers the required heat to the toner image without mechanical contact with the toner image or image carrier substrate, exclusively by electromagnetic radiation for fixation.
In principle, there is a possibility of additionally providing another radiation unit expressly for the black toner, in addition to the radiation units for the other color toners. However, since black toner exhibits a very large wavelength exhibits very good absorption behavior in a very large wavelength range and the wavelength ranges of the radiation for the toner layers with the process colors cyan, magenta and yellow lie within the wavelength range of the black toner, a separate radiation unit for the black toner can preferably be dispensed with. This is then melted by at least one of the other radiation units.
As a further alternative, a digital image carrier device is characterized by the fixation unit has exclusively one radiation unit, the wavelength range of its electromagnetic radiation being variable during melting of the toner image. The wavelength range is varied as a function of the color of the toner image, so that only one of the toner colors absorbs most of the radiation. This means that the toner image is exposed several times in succession to electromagnetic radiation in different wavelength ranges, so that the toner layers are not melted simultaneously, but in succession. Because of this, overheating of the toner image and image carrier substrate can be practically ruled out with a simultaneously simpler and therefore more cost effective design.